Autonumber when choosing text

Hi all,

I would like to generate a unique number(in column A, worksheet BREEAM) when choosing an item from my dropdown in colomn C(see example).
I tried numerous things but with al the options I've tried it works fine untill i'm going to use the sort options.

So what I would like to do is:
- Generate a unique number when there's text in Column C.
- This unique number needs to be generated automatically and checks which is the highest number in column A + 1
- It shouldn't renumber al the items when using the sorting options.

Can anybody point me in the right direction?

Thanks in advance and Kind Regards
Machiel